The invention concerns a process for the production of a screen printing form comprising a screen web, for example of plastic filaments, by coating with an emulsion which in particular is photosensitive. The invention also concerns a screen printing form comprising a coated screen web.
Centuries after it was first used in China the screen printing process has been known in Europe approximately since the 19th Century, a fine-mesh textile cloth or wire mesh material is stretched out in a screen printing frame and covered in the image-free regions by a stencil, so as to be impermeable to ink. Besides manual cut stencils--for example for labelling or writing--nowadays preferably photographically produced direct or indirect stencils are the usual practice; the choice of the kind of stencil adopted--in the case of direct stencils those with emulsion, with direct film and emulsion or with direct film and water--is left to the discretion of the screen printer.
A plurality of steps are usually required to produce a screen printing form. Firstly a screen printing cloth is stretched out over a clamping frame of light metal or alloy, wood or the like, and is glued to the frame in its stretched position. The cloth is then coated with photosensitive emulsion, for example using a coating channel manually or by machine using an automatic coating apparatus. As the coating thereon cannot be produced exactly as far as the inward side of the frame, the remaining surface area must be subsequently sealed off using screen filler. The coated surface is now exposed by means of a copy original (film) corresponding to the print image. The regions of the print image which are not exposed are washed out. The operation of drying the stencil is followed by the retouching operation and the operation of covering over the edges with screen filler.
It has been found inter alia difficult to produce an exact flat layer surface on the screen cloths which are usually produced employing a so-called taffeta weave. Due to the low proportion of solids--usually below 50%--in the emulsion the layer sinks down into the mesh openings of the cloth in the subsequent drying procedure, whereby the surface may exhibit a certain degree of roughness. For certain areas of use, that surface quality may be sufficient, but for higher levels of quality requirement, at least one further coating operation is required.
If a plurality of small frames are to be covered with the cloth--for example for producing print images of small surface area--, the procedure involves the use of a so-called mother frame, and the screen cloth is stretched out thereon; it is held to the mother frame for example by clamping clips which are described in the present applicants' EP 0 650 832 A1. The smaller screen printing frames are put on to a support and the screen web which is held under tension is fitted thereover.
A process for the production of photochemically coated material for screen printing cylinders is disclosed in German laid-open application DE 3441593A. In that procedure a chromium steel wire mesh of suitable mesh size is coated with nickel electrolytically, chemically or by vapour deposition, to such an extent that the points of intersection of the cloth are fixed in such a way that no movement is possible; the stabilized material is then coated with at least one layer comprising a photosensitive emulsion by applying that emulsion to a flat surface and embedding the cloth into the rear side of the layer of emulsion and joining same by applying a further layer of emulsion, so that the result is a flat copying layer on the surface of the screen printing stencil, which is at the printing side.
U.S. Pat. No. 3,834,348A describes a clamping frame--which is variable in width--with a cloth portion fixed therein using clamping elements. Displaceable over the cloth portion is a channel for a viscous fluid or an emulsion, the latter flowing out of adjacent slots in the wall of the channel on to the cloth portion in order to form a coating thereon. The coating is then dried or hardened on the cloth portion.
JP 5 216 239A discloses a container with a clamping frame in which a cloth is coated with a photosensitive film. The fluid is introduced into the container until a desired layer thickness has been produced on the cloth. The fluid is then partially exposed; the unexposed regions are removed after termination of the treatment so as to result in an image.